Fatal error when not converting to speech

Steps to reproduce

1. Enable polly media.
2. Configure a widget on an entity type to support speech synthesis.
3. Save the entity without choosing to convert to speech.

You get an error

TypeError: Drupal\Component\Utility\NestedArray::unsetValue(): Argument #2 ($parents) must be of type array, null given, called in /var/www/html/modules/polly_media/src/Hook/MediaWidgetAlter.php on line 332 in Drupal\Component\Utility\NestedArray::unsetValue() (line 222 of core/lib/Drupal/Component/Utility/NestedArray.php).
Drupal\polly_media\Hook\MediaWidgetAlter::entityToSpeechElementValidate()
call_user_func_array() (Line: 281)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 238)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 118)
Drupal\Core\Form\FormValidator->validateForm() (Line: 593)
Drupal\Core\Form\FormBuilder->processForm() (Line: 326)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 73)
<snip>

Proposed resolution

Add a guard around the offending area.

Remaining tasks

User interface changes

API changes

Data model changes